1. Field of the Invention
The present invention relates to a thermal transfer sheet having two or more types of coloring material layers, a thermal transfer sheet set including a plurality of thermal transfer sheets each having different types of coloring material, and an image forming method that uses the thermal transfer sheet and the thermal transfer sheet set. In particular, the present invention relates to a black-based thermal transfer sheet having a color toning function, a black-based thermal transfer sheet set, and an image forming method using the black-based thermal transfer sheet or the black-based thermal transfer sheet set.
2. Description of the Related Art
Dye sublimation transfer technology has been available as one technique for creating color hard copies. The dye sublimation transfer technology uses a thermal transfer sheet including a substrate film having a heat-resistant lubricating layer on one surface and a coloring material layer on the other surface, in combination with a thermal transfer-receiving sheet including a substrate having an image-receiving layer on at least one surface. According to the dye sublimation thermal transfer technology, a surface of the thermal transfer sheet on which the coloring material layer is formed is arranged to face a surface of the thermal transfer-receiving sheet on which the image-receiving layer is formed, and thermal energy corresponding to an image to be transferred is applied by a thermal head through a heat resistant lubricating layer surface of the thermal transfer sheet so as to allow dye molecules in the coloring material layer of the thermal transfer sheet to migrate to the image-receiving layer surface of the thermal transfer-receiving sheet and to thereby form a transferred image on the thermal transfer-receiving sheet. Since the dye sublimation transfer technology can provide continuous density gradation within one pixel, it is suitable for outputting photographic images.
There has been a demand to create monochromic prints by the dye sublimation thermal transfer technology. Monochromic prints can be obtained by using coloring material layers having the following features:
Method 1: Use of Color Thermal Transfer Sheet
According to method 1, a thermal transfer sheet having yellow, magenta, cyan, and, if necessary, black coloring material layers is used and black/gray tone is obtained by subtractive mixing of these colors. According to method 1, gray-tone prints of a desired hue can be obtained by differentiating the thermal energy distribution ratios for yellow, magenta, and cyan material layers.
However, according to method 1, unintended hue shift occurs in low-density portions of a gray tone print. The term “hue shift” refers to subtle differences in tone between portions with high and low densities that occur during image formation. Whereas a monochromic image should have a constant tone from low-gradation to high-gradation, phase shift causes lack of tone consistency in the monochromic image, which is not preferable. The hue shift is attributable to instability of reproducibility in repeating transfer of dyes by heat application using a thermal head. In other words, according to the method 1, since chroma of the color material layers of three primary colors is originally high, in low-density gray regions, subtle changes in ambient temperature and the temperature of the entire thermal head cause changes in gray tone between different levels of gradation.
Following two methods are also available as methods for obtaining monochromic prints.
Method 2: Use of Monochromic Thermal Transfer Sheet
In method 2, a thermal transfer sheet including an achromatic color material layer is used. Examples of the monochromatic thermal transfer sheet include those having the following coloring material layers, all of which create an achromatic color by imparting absorption over substantially the entire visible light range:    (1) a color layer composed of one type of black-based dye (e.g., refer to Japanese Unexamined Patent Application Publication No. 1-165486 and 2-265790)    (2) a color layer combining a plurality of types of low-chroma dyes having different maximum absorption wavelengths (e.g., see Japanese Unexamined Patent Application Publication No. 4-226393 and 10-86535)    (3) a color layer made achromatic by combining dyes corresponding to three primary colors for subtractive mixing (e.g., see Japanese Unexamined Patent Application Publication No. 1-136787 and 7-304272).
In the case where the coloring material layer (2) is used, a sufficient maximum print density (reflection density of 2 or more) may not be obtained by conducting thermal transfer once. In order to overcome this drawback, coloring material layers of the same color tone are superimposed a plurality of times to increase the maximum print density (e.g., see to Japanese Unexamined Patent Application Publication No. 2-587).
Japanese Unexamined Patent Application Publication No. 7-214804 describes one example of a method for forming an image of a desired color tone in making a monochromic print. The method described in '804 document proposes use of a thermal transfer sheet having a plurality of coloring material layers with symmetric hues of opposite types to obtain a desired color tone.
In method 2, the tone of the gray print is preset and the tone can rarely be changed according to the preference of the user at the site of printing. In order to overcome this drawback, '804 document proposes use of two different types of ink sheets having different hues to obtain a desired tone. However, the selection of the hue is limited to contrastive hues, and the document does not address the problem of hue shift between the low-print-density and high-print-density portions or possible solutions for hue shift.
Another problem with the monochromic thermal transfer sheets (1) to (3) above is that it is difficult to obtain a coloring material layer having a high sensitivity. This is due to the following reasons, which are described according to the composition of the monochromic thermal transfer sheet.
The thermal transfer sheet including a color layer composed of one type of black-base dye described in (1) tends to have a large molecular weight, low dye transfer efficiency, and difficulty in achieving high sensitivity.
As for the thermal transfer sheet having a layer combining a plurality of types of low-chroma dyes having different maximum absorption wavelengths set forth in (2) and the thermal transfer sheet having a layer made achromatic by combining dyes corresponding three primary colors for subtractive mixing set forth in (3), the half value width at the maximum absorption wavelength of each dye used is small in the thermal transfer sheet of (2) and smaller in the thermal transfer sheet of (3), thereby requiring many types of dyes.
Furthermore, the blend ratio of the dye constituting the coloring material layer to the binder constituting the coloring material layer is limited in view of stable dye retention. If the thermal transfer sheet has a dye/binder blend ratio exceeding the upper limit, the dyes may precipitate in the coloring material layer. Because of such restriction, the amount of dye usable for each component becomes more and more limited as the number of types of dyes used increases, and it becomes difficult to transfer a sufficient amount of coloring material (about 2 in terms of maximum print density) by conducting the transfer of the coloring material layer only once. Although this drawback can be overcome by the method described in Japanese Unexamined Patent Application Publication No. 2-587, the problem of lack of choice of tone remains unresolved.
As described above, although the thermal transfer sheets described above have been used to obtain a monochromic print by the dye sublimation transfer technology, it has been difficult to achieve a print density of a practical level, to render the hue shift in the low-density regions less noticeable, and to satisfactorily achieve a desired gray tone.